Present invention is related to the oil industry, particularly an electro acoustic system and associated method for increasing the production capacity of wells that contain oil, and consists of applying mechanical waves in a recovery zone of wells.
The productivity of oil wells decreases over time due to varied reasons. The two main causes of this decrease have to do with a decrease in relative permeability of crude oil, thus decreasing its fluidity, and progressive plugging of pores of a reservoir in a well bore region of a well due to accumulation of solids (clays, colloids, salts) that reduce the absolute permeability or interconnection of the pores. Problems associated with the aforementioned causes are: plugging of the pores by fine mineral particles that flow together with fluid to be extracted, precipitation of inorganic crusts, paraffin and asphaltene decantation, clay hydration, invasion of mud solids and mud filtration and invasion of completion fluids and solids resulting from brine injection. Each one of the reasons mentioned above may cause a decrease in the permeability or a restriction of flow in the region surrounding the well bore.
A well (FIG. 1) is basically a production formation lined with a layer of cement 19 and a case 10 that in turn holds a series of production tubes 11 placed coaxially within it. The well connects an oil reservoir, which has an appropriate permeability that allows the fluids produced in the formation 12 to flow through perforations 14 and/or holes 13 in the lining of the well, providing a route within the formation 12. The tubes 11 provide an outlet for the fluids 18 produced in the formation. Typically there are many perforations 14 which extend radially on the outside from the lined well. The perforations 14 are uniformly spaced out on the lining where it passes through the formation 12. Ideally, the perforations are placed only in the formation 12, so the number of these depends on the thickness of the formation 12. It is quite common to have nine to twelve perforations per meter of depth in the formation 12. On the other hand the perforations 14 extend in every longitudinal direction, so there are perforations 14 that can extend radially at an azimuth of 0° while additional perforations 14 are placed each 90° so as to define four groups of perforations 14 around the azimuth.
The fluids of the formation 12 flow through the perforations 14 entering the lined well. Preferably, the well is plugged by some sealing mechanism, such as a packer 15 or bridge plug placed beneath the level of the perforations 14. The packer 15 connects with the production tube 11 defining a compartment 16 into which the fluid produced from the formation 12 flows, filling the compartment (16) and reaching a fluid level (17). The accumulated fluid 18 flows from the formation 12 and may be accompanied by variable quantities of natural gas. In summary, the lined compartment accumulates oil, some water, natural gas and also sand and solid residues. Normally the sand settles in the bottom of the compartment 16. The fluid produced from the formation 12 may change phase in the event of a pressure reduction about the formation 12 which permits lighter molecules to vaporize. On the other hand, the well may also produce very heavy molecules.
After a period of time, the pathways through the perforations 14 extended within the formation 12 may clog with “fines” or residues. This defines the size of the pore that connects with the fluid within the formation 12, allowing it to flow from the formation 12, through the cracks or fissures or connected pores, until the fluid reaches the interstitial spaces within the compartment 16 for collection. During this flow, very small solid particles from the formation 12 known as “fines” may flow, but instead tend to settle. Whereas the “fines” may be held in a dispersed state for some time, they can aggregate and thus obstruct the space in the pore reducing the production rate of fluids. This can become a problem which feeds upon itself and results in a decrease in production flow. More and more “fines” may deposit themselves within the perforations 14 and obstruct them, tending to prevent even a minimum flow rate.
Even with the best production methods and the most favourable extraction conditions, a percentage higher than 20% of the crude oil originally existing within the reservoir typically remains behind.
The periodic stimulation of oil and gas wells is made using three general types of treatment: acidification, fracturing and treatment with solvents and heat. Acidification involves the use of HCl and HF acid mixtures which are injected into the production zone (rock). The acid is used to dissolve the reactive components of the rock (carbonates and clay minerals and, to a lesser extent, silicates) and thus increase its permeability. Additives such as reaction retardants and solvents are often added to enhance the performance of the acid at work. While acidizing is a common treatment for stimulating oil and gas wells, it clearly has some drawbacks, namely the high cost of chemicals and waste disposal costs involved.. The acids are often incompatible with the crude oil and may produce thick oily residues within the well. Precipitates formed after the acid is spent may often be more harmful than the dissolved minerals. The depth of penetration of the live acid is usually less than 5 inches.
Hydraulic fracturing is another technique commonly used for stimulation of oil and gas wells. In this process, great hydraulic pressures are used to create vertical fractures in the formation. The fractures may be filled with polymer plugs or treated with acid (in carbonates and soft rocks) to create conduits within the well that allow the oil and gas to flow. This process is extremely expensive (by a factor about 5 to 10 times more than the acid treatment). In some cases the fracture can extend into areas with water, increasing the amount of water produced (undesirable). Such treatments extend many hundreds of feet away from the well and are more commonly used in rocks with a low permeability. The ability to place polymer plugs successfully in all the fracture is usually limited and problems such as fracture closures and plug (proppant) crushing can severely deteriorate the productivity of hydraulic fractures.
One of the most common problems in mature oil wells is the precipitation of paraffin and asphaltene within and around the well. Steam or hot oil is injected into the well to melt and dissolve the paraffin in the oil, making everything flow to the surface. Organic solvents (such as xylene) are often used to remove asphaltenes, whose fusion point is high and are insoluble in alkanes. The steam as well as the solvents are very expensive (solvents more so than the steam) in particular when treating marginal wells that produce less than 10 bbls of oil per day. It should be noted that there are more than 100,000 of such wells only in the state of Texas and probably many more in other states in the USA.
The prime limitation for use of steam and solvents is the absence of mechanical agitation, required to dissolve or maintain in suspension the paraffin and asphaltenes.
In U.S. Pat. No. 3,721,297 to R. D. Challacombe, a tool is proposed for cleaning the wells by pressure pulses, whereby a series of explosive modules and gas generators are chain interconnected in such a way that the lighting of one of them triggers the next in one succession.
The explosions create shock waves that allow cleaning of the wells. This method has clear drawbacks, such as the potential danger of damaging high pressure oil and gas wells with explosives. This method is made unfeasible by the added risk of fire and lack of control during the treatment period.
The U.S. Pat. No. 3,648,769 to H. T. Sawyer describes a hydraulically controlled. diaphragm that produces “sinusoidal vibrations in low sonic range”. The waves generated are of low intensity and are not directed or focused at the rock face. As a consequence, most of the energy propagates along the borehole.
U.S. Pat. No. 4,343,356 to E. D. Riggs et al. describes an apparatus for treating surface boreholes. The application of high voltage produces the generation of voltage arcs that dislodge the scale material from the walls of the well. Amongst the difficulties of this apparatus is the fact that the arc cannot be guided continuously, or even if any cleaning is accomplished at all. Additionally the subject of security remains unsolved (electrical and fire problems).
Another hydraulic/mechanical oscillator was proposed by A. G. Bodine (U.S. Pat. No. 4,280,557). Hydraulic pressure pulses created inside an elongated elastic tube are used to clean the lined walls of the wells. This system also suffers from low intensity and limited guiding.
Finally, a method for removing paraffin from oil wells was proposed by J. W. Mac Manus et al. (U.S. Pat. No. 4,538,682). The method is based on establishing a temperature gradient within the well by introducing a heating element into the well.
It is well known that the oil, gas and water wells, after some time of operation become obstructed and the fluid discharge declines, such that it becomes necessary to regenerate wells. The mechanical, chemical and conventional techniques for regenerating wells are the following:
Intensive rinsing
Shock pumping
Air treatment
Dissolution of sediments with hydrochloric acid or other acids combined with other chemicals.
High water pressure hosing
Injection of CO2
Generation of pressure shocks by use of explosives
These methods work with harmful chemicals, or work at such high power that they may be a risk to the structure of the well.
There exist a great number of effects associated to the exposure of solids and fluids to ultrasound fields of certain frequencies and power. Particularly in the case of fluids, it is possible to generate cavitation bubbles, that consists in the creation of bubbles from gasses dissolved in the liquid or from the phase change of this last. Other phenomena associated are degassing of liquid and the superficial cleaning of solid surfaces.
Ultrasound techniques have been developed with the aim of increasing the production of crude from oil wells. U.S. Pat. No. 3,990,512 to Arthur Kuris, titled “Method and System for Ultrasonic Oil Recovery”, divulges a method and system for recovering oil by applying ultrasound generated by the oscillation produced while injecting high pressure fluids and whose aim is to fracture the reservoir so as to produce new drainage canals.
U.S. Pat. No. 5,595,243 to Maki, Jr. et al. proposes an acoustic device in which a set of piezoceramic transducers are used as radiators. This device presents difficulties in its fabrication and use, as it requires asynchronic operation of a great number of piezoceramic radiators.
U.S. Pat. No. 5,994,818 entitled “Device for Transferring Ultrasonic Energy into a Liquid or Pasty Medium”, and U.S. Pat. No. 6,429,575, titled “Device for, Transmitting Ultrasonic Energy to a Liquid or pasty Medium”, both belonging to Vladimir Abramov et al., propose an apparatus consisting of an alternate current generator that operates in the range of 1 to 100 kHz for transmitting ultrasonic energy and a piezoceramic or magnetostrictive transducer that emits longitudinal waves, which a tubular resonator coupled to a wave guide system (or sonotrode) transforms in turn to transversal oscillations in contact with the irradiated liquid or pasty medium. Notwithstanding, these patents are designed for use in containers of very big dimensions, at least in comparison with the size and geometry of perforations present in oil wells. This presents limitations of dimension as well as in transmission mode if increasing production capacity of oil wells is desired.
U.S. Pat. No. 6,230,799 to Julie C. Slaughter et al., titled “Ultrasonic Downhole radiator and Method for Using Same”, proposes a device using ultrasonic transducers made with Terfenol-D alloy, placed in the bottom of the well and fed by an ultrasound generator placed at the surface. The disposition of the transducers on the axis of the device allows emitting in a transversal direction. This invention poses a decrease in viscosity of hydrocarbons contained inside the well through emulsification when reacting with an alkaline solution injected into the well. This device considers surface forced fluid circulation as a cooling system, to guarantee irradiation continuity.
U.S. Pat. No. 6,279,653 to Dennos C. Wegener et al., titled “Heavy Oil Viscosity Reduction and Production”, presents a method and device for producing heavy oil (API gravity lower than 20) by applying ultrasound generated by a transducer, made with Terfenol alloy, attached to a conventional extraction pump and fed by a generator placed at the surface. This invention also considers the presence of an alkaline solution, like a watery solution of Sodium Hydroxide (NaOH) for generating an emulsion with crude in the reservoir of lesser density and viscosity, and thereby making the crude easier to recover by pumping. Here, a transducer is placed in an axial position so as to produce longitudinal emissions of ultrasound. The transducer connects to an adjoining rod that acts as a wave guide (or sonotrode) to the device.
U.S. Pat. No. 6,405,796 to Robert J. Meyer, et al., titled “Method for Improving Oil Recovery Using an Ultrasound Technique”, proposes a method for increasing the recovery of oil using an ultrasonic technique. The proposed method consists of the disintegration of agglomerates by ultrasonic irradiation posing the operation in a determined frequency range with an end to stimulating fluids and solids in different conditions. The main mechanism of crude recovery is based on the relative movement of these components within the reservoir. All the preceding patents use the application of ultrasonic waves through a transducer, fed externally by an electric generator, whose transmission cable usually exceeds a length of 2 km. This brings with it the disadvantage of losses in the transmission signal, which means that a signal has to be generated sufficiently strong so as to allow the appropriate functioning of the transducers within the well, because the amplitude of the high frequency electric current at that depth decreases to a 10% of the initial value.
As the transducers must work with a high power regime, an air or water cooling system is required, presenting great difficulties when placed inside the well, meaning that the ultrasonic intensity must not be greater than 0,5–0,6 W/cm2. This quantity is insufficient for the purpose in mind as the threshold for acoustic effects in oil and rocks is 0,8 to 1 W/cm2.
RU Patent No. 2,026,969,belonging to Andrey A. Pechkov entitled “Method for Acoustic Stimulation of Bottom-hole zone for producing formation, RU No 2,026,970 belonging to Andrey A. Pechkov et al., entitled “Device for Acoustic Stimulation of Bottom-hole zone of producing formation”., U.S. Pat. No. 5,184,678 to Andrey A. Pechkov et al., entitled “Acoustic Flow Stimulation Method and Apparatus”, divulge methods and devices for stimulating production of fluids from inside a producing well. These devices incorporate as innovative element an electric generator together with the transducer, both integrated at the bottom of the well. These transducers operate in a non continuous regimen allowing them to work without requiring an external cooling system.
A suitable stimulation of the solid materials requires efficiency in the transmission of the acoustic vibrations from the transducers to the rock of the reservoir, which in turn is determined by the different acoustic impedances inside the well (rocks, water, walls, and oil, amongst others). It is well known that the reflection coefficient is high in a liquid-solid interface, which means that the quantity of waves passing through the steel tube will not be the most adequate to act in the interstices of the orifices that communicate the well with the reservoir.